Does your son MIND wearing diapers at night? Either the pullups/big kid kind or the old fashioned kind with the tabs on the side? Is it just that you don't want to change them? In some ways I was the odd parent because I always thought it was a LOT easier to deal with diapers than to watch my child like a hawk to see if they were dancing or standing in some odd way to tell if they had to use the toilet. It was also easier to change a diaper than to change and wash wet clothing and esp wet sheets. So my kids had to earn the right to wear cloth underwear and they each did it on very different schedules. Each was dry at night at different times also. I did not EVER get a child up in the middle of the night to go to the bathroom. If they were not ready to be dry at night, then they just were not ready.
It was more about being physically ready and realizing that not every body is ready to be dry at night at the same age than it is for any other reason, at least for me. Also it was about having more to do than to pay that much attention to someone else's bowel and bladder habits.
I will say that part of my opinions were formed because we had a friend who insisted that each of her children were 'potty trained' by 18 to 20 months, but all this meant was that she watched her kids every single second and if they moved this way or that way then she rushed them to a bathroom or one of the many, many potty chairs (literally in every room in her home - yuck!), and fought with them until they sat there until they did something. She had compliant children and she ended up fighting with the youngest one until he finally figured out what she wanted when he was six.
So she spent four years fighting over his toilet habits. He is autistic and truly didn't have a clue what she wanted for over half that time, and few people would babysit him more than 1 or 2 times. Her mother was her daycare provider and what my friend didn't know was that at her mom's house was that the boy wore diapers. Grandma knew he was not ready and she didn't want her house ruined by accidents. She didn't want to punish him over it because it does no good.
Please don't punish him for accidents. Making him clean up after himself isn't a punishment, but other than that, all you are doing is creating conflict and anger over something that he cannot really help. At least not at night. The odds that he is awake and doing this to be defiant are very slim. Chances are that he isn't fully awake, and likely has no real idea what is expected, regardless of what he is saying.
I am a FIRM believer that many potty training issues will work themselves out with maturity and a hands-off policy by mom and dad. Just let him use diapers/pullup type things at night. You will all get more/better sleep.
You might want to be aware that it is common for kids on medications for adhd to sleep FAR more deeply than kids who are not on adhd medications. If they are fully potty trained for years before they start medications, it is still common for them to have accidents on a regular basis once they start adhd medications, esp stimulants. Their brains just crash so hard that waking them even enough to use the toilet is very difficult. For children who are not yet trained, it is going to take longer to become toilet trained at night. The child cannot help it and is not doing it on purpose.
Have you spoken to your son's Occupational Therapist (OT) about any sensory issues with using the toilet? Have you asked your son why he is uncomfortable using the toilet? Is he using the regular toilet or a potty chair? Does he have a smaller seat more his size to sit on, or does he sit on the regular adult size toilet seat? Or does he stand and pee like Daddy does?
Is he willing to tell you, his dad or a male person why he doesn't want to pee in the potty? I ask for a reason? I remember being flat out TERRIFIED of the adult potty as a kid. My older brother told me that if I fell it, I would be trapped and it would eat me. That seems silly to me NOW, but as a small child, it was truly scary. I would not tell anyone that for ANYTHING. I can remember being unwilling to tell anyone because either it would be true and we had a monster in our house or it wouldn't and my brother was just that mean and we had a different monster in our house. But finding out what the problem is means finding a solution. I just couldn't see a way out of using the toilet.
As for asking about the seats, an adult seat can seem VERY unstable to sit on to a child. You literally feel like you will fall in. I remember that feeling well. I used to have to brace my hands behind me to stay sitting on the toilet without falling in. I also needed a stool because otherwise my feet were so far off the floor. Sometimes you can figure out the reason behind the problem and fix it without a lot of problems. I will say that finding that reason can be challenging with kids with autism, but there is a reason why behind every problem when a child has autism, at least there is a why at least 99.9% of the time.
It may be that your son won't talk about this with you or Dad. When it came to this topic, one of my kids would NOT talk to me or my hubby (his father) no matter what. He would talk to my mother or father though. Sometimes it had to be long distance, but we used that. My parents were great sports about it, and it was appreciated, esp when he would get that look that told us clearly that he thought we were idiots who didn't know what we were talking about. Finding the person that your son will speak to about it will help. Then you have to listen to him. Until then, I would go to rewards for being dry (use whatever he loves most that you restrict the most - candy, video games, whatever) and diapers. Take the pressure off and stop punishing him - they call them accidents for a reason, they don't call them defiants, do they?